1. Field of the Invention
This invention pertains to the general field of optical filters and, in particular, to a novel approach for fine tuning high-performance etalon filters.
2. Description of the Prior Art
Etalons are well known optical devices that consist of two reflective surfaces parallel to one another and spaced apart by a predetermined optical length. They may consists simply of a solid parallel plate (so called “solid etalons”) or of two plates with an air gap between them that defines a cavity (so called “air-spaced etalons”), as illustrated in FIG. 1. When illuminated with a broadband collimated light, etalons produce a transmission beam and a reflection beam with periodic spectra characterized by very narrowband spikes of wavelength determined by the physical properties and dimensions of the etalon. A typical etalon transmission spectrum is illustrated in FIG. 2. With reference to air-spaced etalons, in particular, the specific center wavelength □′ of the passband (the spectral spike) and the period between spectral spikes (commonly referred to in the art as channel spacing or free spectral range, FSR, of the device) are a function of the optical length of the etalon's cavity.
In particular, referring for example to the etalon 10 and the intensity spectrum 12 of FIGS. 1 and 2, respectively, minor changes in the optical length L of the cavity 14 will cause a shift of the periodic spectrum along the wavelength axis, as indicated by arrows 16. As is well understood by those skilled in the art, varying the optical length of the cavity also produces a change in the free spectral range of the etalon.
These properties of etalons are very advantageous for many optical applications. In particular, etalons are used as high-performance filters to isolate light of a very a precise frequency, as may be needed for a particular application. In telescopic astronomy, for instance, such filters are particularly useful for observing objects at specific wavelengths. Since the exact wavelength of each peak is a function of the exact optical length L of the cavity, it has been most important in the art to build etalon filters with precise and uniform spacing between the two plates (18,20) constituting the etalon (FIG. 1). To that end, very precisely machined spacers 22,24 of equal thickness L′ are used, typically uniformly distributed around the annular periphery of the plates in a sufficient number to separate the plates and produce a cavity of uniform optical length L. (It is noted that L′ is the physical cavity length corresponding to the desired optical path length L, the two quantities being related by the equation L=nL′, where n is the index of refraction of the medium in the cavity.).
In practice it has been difficult and expensive to achieve the desired degree of perfection because of the very narrow tolerances (in the order of nanometers) required for the level of performance associated with astronomy applications. U.S. Pat. Nos. 6,181,726 and 6,215,802 disclosed several advances over the prior art whereby the uniformity of the etalon's optical length was improved. According to one approach described in the patents, all the spacers used to form the etalon are selected from a common local area of a spacer substrate produced by standard-precision optical manufacturing techniques. It was discovered that, as a result of this selection, the spacers tend to have substantially more uniform thickness and, therefore, they produce a more uniform etalon cavity. According to another, complementary approach, an additional spacer from the same local substrate area is used at the center of the etalon, thereby providing a correction to plane deformations produced by the optical contact of the peripheral spacers with the etalon plates.
While the techniques described in these patents provide a significant improvement over the etalons previously known in the art, they are very labor-intensive and therefore expensive to practice. In addition, the resulting etalons, while more uniform in the optical length of the cavity, are not necessarily tuned to the precise desired wavelength. Therefore, there is still a need for an extremely accurate and relatively inexpensive way of controlling the uniformity as well as the optical length of the etalon cavity. The present invention provides simple solutions to that end.